Astronomy

Lumicon Oiii Filter



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I have been an avid astronomer for a number of years now, when I first started observing the heavens I concentrated on moons and planets within our solar system. But it wasn't long before my curiosity (and experience) grew enough to leave the confines of the cosmic incubator and go in search of larger more distant quarry.

There are millions of wonders in deep space, split into a handful of categories, two such categories are planetary nebula and emission nebula.

Planetary nebula= The intricate sphere of ionized gas given off by a dying sun.

Emission nebula= A cloud of gas much larger than a planetary nebula, in effect an emission nebula is a stellar nursery, when a certain critical mass is met a portion of the dust cloud collapses in on itself forming a baby star. Earth's sun would have been one of some 20 stars conceived in such a cloud, although in the past 6 billion years since they were born Earth's cousins have become scattered throughout our galaxy. It's interesting to think that about 20 stars with an almost identical chemical composition to our sun are out there in the milky way just waiting to be rediscovered.

These are the only two groups to benefit from an Oiii filter (in fact galaxies, globular clusters and even ordinary stars become much fainter and harder to see with the filter.)

An Oiii filter works by allowing only a narrow band on the colour spectrum to pass through it and into the observers eye, it is a wavelength of light that is given off in large amounts by most planetary and emission nebulae. The end result is an object that seems brighter and more contrasted with the sky, in truth the object has been dimmed slightly but the sky around it has been dimmed to a higher degree making the object clearer and more defined.

What is supposed to make lumicon's filter stand out from the rest is that it allows a smaller amount of the light spectrum through it, corresponding more exactly with the precise wavelengths given off by the nebulae, blocking out more unwanted stray light and leaving you only with the light from the objects you're observing.

When reading reviews and comments on various astronomy forums and magazines I found that the results could be quite stunning, and as I live under fairly light polluted skies (as do most of us these days) any help I could get in gaining contrast in some of my favourite nebula would be very welcomed indeed.

I ordered the filter for around £90 and received it a couple of days later, the filter is about an inch in diameter (actually it's 1.25 inches so it can be threaded into any standard 1.25 inch eyepiece barrel) . The lens is highly reflective (of course) and looks like a tiny mirror surrounded with a black metal edge.

Now onto the field test, after waiting for a week for the clouds to clear (it's a common curse in the world of astronomy, whenever new equipment is bought it is always followed by a prolonged period of cloudy nights, typical) after what seemed like an eternity, I finally got out to use it.

First thing to note is the filter screws in nicely to all the eyepieces in my collection, they fit snuggly with no wobble and so no fear of them tumbling out and destroying your delicate primary.

I first observed NGC2392 (The Eskimo Nebula), a bright little planetary at magnitude 8.6 (with a surface brightness of 6.8), its similar in size to Jupiter at around 42 arcseconds. The nebula gets it's name from it's appearance when viewed through large telescopes, it sort of looks like a little Eskimo with a furry hood.

Without the filter only the inner shell of the nebula is visible, with the filter in place the fainter outer shell becomes visible but only just, right on the limits using averted vision.

Next up was the blue snowball (NGC7662). A large planetary neb at 2 arcminutes and 12 arcseconds, to help visualise it, that's about 8% the apparent diameter of the full moon. It's slightly brighter than the Eskimo (with a surface brightness of magnitude 5.6) but it's spread over more sky so appears ever so slightly fainter. The nebula did appear a little brighter with the filter but no further details could be seen. Not quite as good a result as the Eskimo.

Then on to the Ring in Lyra. This is arguably the most famous PN and appears through medium aperture telescopes as a small ring of smoke. This was the nebula I was expecting the most from but actually turned out getting the least from. It appeared almost identical with or without the filter, a little disappointed by M57 to be honest.

Now onto another famous nebula, although the first emission nebula of the night, m42. This is a nebula that's easy to see with the naked eye under even moderately light polluted skies. It's a whopping 1.6 degrees (taking up about 1.5 times the area of sky as the full moon).This nebula on it's own nearly justified the £90 for the filter. At 500x magnification I could actually see texture within the swirling mass of interstellar dust, the detail was exquisite and nearly blew me away. Definitely a high point for this filter.

In total since purchasing the filter I have used it on a wide number of nebulae, with varying levels of success. But the highlight is still the jaw dropping views of the orion nebula (M42), a view I reckon will be hard to top.

As we all know, nothing in this world is ever perfect, this is indeed true of the Lumicon Oiii, I should discuss some grievances with the filter as well as all the beautiful views I've had.

My first gripe is the way the filter dims starlight (although I should say this occurs in all Oiii filters, it's just more pronounced in the Lumicon), when operating at high magnification it's very useful to have a visible star near to the object you're looking at, it's an easy way to achieve a perfect focus. With the disappearance of all but the brightest stars, getting good focus becomes more difficult and a lot more tedious.

Secondly (apart from my views of M42) I feel the filters results were infinitely over hyped by almost all the reviews I read, the results were noticeable but in my opinion they were certainly not worth the £90 price tag. Objects that were described as being unrecognisably transformed by "this stunning filter", were in fact only marginally improved. To put it in a nutshell I would happily have paid a third of the price for the filter and no more.

At the end of the day only you can know for sure whether the views you receive will be worth what you pay for them. All I can do is give my account and hope that gives you a little insight to help you with your upcoming purchase. I can offer some further advice by saying that generally the bigger your scope the better the filter will work, also it tends to hit a sweet spot between 2mm and 4mm of exit pupil (with 4mm providing the best views).

Can I recommend the Lumicon as a good filter? Yes, but I do find its cost out-ways the improvement to views. Can I recommend it over other Oiii filters? Perhaps not, my reasons are twofold. Firstly you can get similar results for around half the price, and secondly some of the less narrow Oiii filters don't dim the stars to quite the same extent, making it easier to get fine focus.

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